In recent years, the increasingly widespread use of display device alternatives to the cathode ray tube (CRT) has driven the demand for large-area electronic arrays. In particular, amorphous silicon and laser re-crystallized polycrystalline silicon (poly-silicon) are used to drive liquid crystal displays commonly used in laptop computers. However, fabricating such large-area arrays is expensive. A large part of the fabrication cost of the large-area arrays arises from the expensive photolithographic process used to pattern the array. In order to avoid such photolithographic processes, direct marking techniques have been considered an alternative to photolithography.
An example of a direct marking technique used in place of photolithography involves utilizing a xerographic process to deposit a toner that acts as an etch mask. However, toner materials are hard to control and difficult to remove after deposition.
Another example of a direct marking technique involves “digital lithography” in which a droplet source including, for example, an inkjet printhead, is used to deposit a liquid mask onto a substrate in accordance with predetermined printing data. A problem with digital lithography is that inkjet printing of functional devices is susceptible to several defect creation processes during the printing operation: misdirected ejection, ejection failure, droplet/spot size variation, alignment error, etc. In most device printing applications, single defects, depending on their nature, will result in a device that will not function to specifications.
It is highly desirable to develop robust digital lithography methods that maximize yields. Currently, the method of quality control for micro electronic and optical pattern formation by digital lithography involves post-printing inspection of the pattern after the entire substrate is patterned. Feedback is, at best, available to the process for subsequent pattern formations, but such feedback information is essentially useless when applied to patterns printed onto large flexible sheets, which often exhibit random surface distortions due to stretching or local stresses. In rare instances, post-processing may be attempted to correct printing errors. However, such corrections are performed well after deposited materials have gone through a phase change (i.e., assumed a solid form), thereby producing inferior correction results because the corrective liquid mask may not adhere well to the already-solid mask material.
What is needed is a digital lithography system that includes real-time quality control in the form of real-time monitoring and modification of the digital lithography process in order to improve the positional accuracy of printed features, and to improve the quality of corrected printed structures on flexible and rigid substrates.